Downhill Downshifting and Engine Braking

[akadeadeye]

I know this topic has probably been "exhausted" (har, har) but I want my understanding to be clear.

I have an Engine Brake and an Allison 3000 transmission.  I use the engine brake quite often when descending hills and mountains but would like to learn to use downshifting as well.  Can the two be used at the same time?  If I am applying the engine brake in either low or high mode, do I even need to worry about also manually downshifting?  If I downshift to, say, 3, and reach a levelling off part of the mountain will it automatically upshift or do I have to do that?

Don

[hermanmullins]

Don, 

When I apply my Jake Brake, my transmission go to 2nd or second gear. My coach has 400 ISL with the Allison 3000 and a Jake Brake.

When I am on a descending hill or mountain I sometimes still have to feather the brakes. I just work them all together.

Herman 

[bm02tj]

The program down shift is normally to fourth  if you manually shift to third it will stay in third unless the motor is over speed then it will shift up to protect engine 

[wayne77590]

Mine is an Exhaust Brake and it hold well on most simple downgrades.  It will drop the transmission a gear dependent on the speed I am going and typically to 4th gear as the set point and as the speed decreases to a safe transmission drop speed.. However, on long downhill grades the coach will slowly increase speed and I either have to stab the brake to bring it below what I consider the optimum speed for the grade, or down shift to the gear that will maintain control. 

When I have to downshift and upon leveling out the coach will stay in the gear that it has been downshifted to. I need to shift back up to 4, 5 or 6th gear and usually it is to 6th gear and let the system do what it is supposed to do.  If I am using just the Exhaust Brake everything is "automatic."  I leave my exhaust brake on all the time.

[jleamont]

Since we are on the topic; what is the highest RPM your coach will rev to with the engine brake on slowing down. Mine will top out after it downshifts itself to 2200 RPM which makes my hair stand up. I think it's to high. Is this common with everyone else?

[wolfe10]

Most engines have two different "how fast can I safely rev the engine":

Under load, aka governed RPM.

Overspeed/under no load/coasting/engine braking.  This is usually a couple of hundred RPM higher than the safe speed while the engine is working hard.

Give your engine manufacturer a call with you engine serial number to determine these numbers for your engine.

I suspect (please confirm) that 2200 overspeed/no load is not a problem for your ISL.

 

Cummins 800 343-7357

[wayne77590]

2nd gear downhill will push me to 3000 rpm.When I catch it I stab the brakes and slow down.

[tbutler]

I have the ISL 400 and Allison 3000.  When I use the engine brake (low or high), the transmission will select 4th gear but won't shift if the speed is too high.  I don't manually downshift, I let the Allison handle that.  If I want a downshift I'll apply the service brakes enough to bring the speed down and the Allison will downshift on it's own.  In ideal conditions, I'll apply the engine brake before the speed becomes too high for the Allison to downshift.  If I want a downshift to 3rd gear, I'll apply service brakes and slow the coach until the Allison shifts down to third gear.

The Allison 3000 as it is set up on our coach will shift up to the next higher gear when the engine RPM reaches 2500.  If the RPM is approaching that level and I don't want the Allison to shift to the next higher gear, I'll apply the service brakes to bring the RPM down to something like 2200 or 2300 and then let the engine brake handle the descent until it approaches 2500 again.

[manholt]

Cummins 425 Allison 3000.  When descending I use the engine brake and the Allison automatically downshifts to 23 and stays there until I exit the EB mode, then the Allison automatically change back to whatever gear I was in prior to hill, usually 5 or 6.  RPM stays at 2200 to 2400.  Per Cummins, perfectly normal.  On long down hills, I switch back and forth some times.

I never drive with my brake on, as it will affect the cruise control.  My owners manual on 4 different DP's have said, don't drive with engine brake on in CC.

 

Carl C.

[jleamont]

Brett, good advice as usual. I called them last week, he told me max 1800. I explained my question again, he put me on hold came back and told me he and his supervisor couldn't locate any documentation. That was the second time I got either no or bad information from them.

[wolfe10]

7 hours ago, Wayne77590 said:

2nd gear downhill will push me to 3000 rpm.When I catch it I stab the brakes and slow down.

We need to be careful here-- some "apples vs oranges" comparisons.

Different engines have different RPM limits.  Basically, the larger the displacement, the longer the stroke, the slower the engine turns.  "Piston feet/minute" has always been the prime determinant.

No different than a gasoline engine.  Safe operating RPM for the GM 8.1 liter V8 is lower than the Ford 6.8 liter V10.

[jleamont]

I forgot to mention, they did make me feel better when Cummins told me that no factory software will allow that engine to over speed, if the Allison would do it my engine light would come on. What the RPM is when over speed occurs was my original question. It just seems too high for my liking. I can only hope Allison and Cummins figured all of this out before and both are programmed to stop that from happening.

Someone has to know the answer.

[akadeadeye]

10 hours ago, TBUTLER said:

I have the ISL 400 and Allison 3000.  When I use the engine brake (low or high), the transmission will select 4th gear but won't shift if the speed is too high.  I don't manually downshift, I let the Allison handle that.  If I want a downshift I'll apply the service brakes enough to bring the speed down and the Allison will downshift on it's own.  In ideal conditions, I'll apply the engine brake before the speed becomes too high for the Allison to downshift.  If I want a downshift to 3rd gear, I'll apply service brakes and slow the coach until the Allison shifts down to third gear.

The Allison 400 as it is set up on our coach will shift up to the next higher gear when the engine RPM reaches 2500.  If the RPM is approaching that level and I don't want the Allison to shift to the next higher gear, I'll apply the service brakes to bring the RPM down to something like 2200 or 2300 and then let the engine brake handle the descent until it approaches 2500 again.

Tom,  This is the info I was looking for.  Thanks for your reply.  All good comments by others as well.  I have just never manually downshifted, always relying on the engine brake in either high or low setting, but wondering if combining the two is advisable.  I doubt we will encounter any grades steeper than 9-10% and slow and easy, plus the engine brake has worked before, but I do find myself using service brakes a little more than I want to so I was wondering if I should be downshifting as well.

 

Don

[manholt]

Where do you find a 9-10% grade?  I do all time 4 wheeling, but have never seen one on the highways.

[wayne77590]

If you decide to camp in Palo Duro Canyon, TX, it is 10% grade for about 2 miles down to the bottom.

[manholt]

Did not realize it was that steep.  When my DW drove down a mountain...we where there in 1983...I found something else to do !

Carl C.

[unw1red]

1 hour ago, Manholt said:

Where do you find a 9-10% grade?  I do all time 4 wheeling, but have never seen one on the highways.

Teton Pass on WY-22 between Idaho and Wyoming is 10% grade for over 7 miles. Barely made it to the top last week on way to Jackson Hole. 

[jleamont]

For those of you that have a two stage engine brake (hi/low) with the Cummins ISL, what operating characteristic do you notice on low vs high? Does it still down shift the transmission but with less resistance on low?

Just curious, I still haven't found the time to bring the lap top from work to home so I can reprogram it so low works, yes I have the switch but the previous owner had the programming changed so it only works on high in either position so I was told by Monaco anyway still baffles me....

[wolfe10]

20 minutes ago, jleamont said:

For those of you that have a two stage engine brake (hi/low) with the Cummins ISL, what operating characteristic do you notice on low vs high? Does it still down shift the transmission but with less resistance on low?

Yes, on either high or low (3 or all 6 cylinders) engine compression brake, the Allison does downshift toward the "preselect gear"/the gear indicated in the only window of single window shift pads or the left window of two window shift pads.

When using the compression brake to come to a stop, I always use high.  I use low only for fine-tuning speed of descent on long grades-- just gives one more option on amount of braking HP.

[jleamont]

Thanks Brett. This is one of the items on my list that is not critical but annoying. Other than my fuel issue which I believe is behind me (last fuel sample report due in tomorrow) my critical list is behind me. Now onto the annoying items like this.